In recent years, the technology for producing a LCD with large size or fine image has been improved. By selecting the liquid crystal materials with suitable dielectric anisotropy, a LCD with quick response time can be provided, and the image-retention problems can be eliminated at the same time.
In a LCD module, it is desirable to increase the charge holding rate. On the other hand, it is also desirable to increase the resistivity of the liquid crystal materials to restrict the local current flow through the liquid crystal materials. Therefore, to obtain high resistivity, it is desirable that the fineness of a liquid crystal material be substantially without any impurity. Some refining processes may be accomplished when producing the liquid crystal materials However, the resistivity of a liquid crystal material is not proportional to the charge holding rate, when it is higher than a predetermined level. The correlation between the resistivity and the charge holding rate may also depend on the dielectric properties derived from the resistivity, or the chemical compound structure of a liquid crystal material.
In general, the greater dielectric properties the liquid crystal material has, the more ionic impurities that may be released in the liquid crystal materials. Thus, the local current can flow through the liquid crystal materials easier. Accordingly, the existence of the ionic impurities in the liquid crystal materials can result in the decrease of the charge holding rate. It is desirable to provide an improved method for obtaining a finer liquid crystal material without ionic impurities.
The low-voltage liquid crystal materials generally contain ionic impurities, such as fluorine ion and/or cyanogen ion, dissolving in the liquid crystal materials to make the larger dielectric anisotropy and larger polarity. The liquid crystal materials with cyanogen ions may release more ionic impurity. In contrast, the liquid crystal materials with fluorine ions can prevent the charge holding rate from degradation as well, even if the dielectric anisotropy was increased. The liquid crystal materials with fluorine ions can prevent the charge holding rate of the liquid crystal materials from degradation, even the dielectric anisotropy of the liquid crystal materials is increased. Briefly, the impurity substituted by fluorine ions of liquid crystal materials can reduce its resolvability within the liquid crystal materials, so that the fluoro liquid crystal materials can be used for preventing the charge holding rate of the liquid crystal materials from degradation.
In order to improve the performance of the LCD, high-dielectric liquid crystal materials may be used. However, compared to other materials, when the liquid crystal materials with high dielectric properties are used, the high-electric liquid cyanogen crystal materials tend to release ionic impurity more easily than the other liquid crystal materials. Recently, liquid crystal materials with high dielectric anisotropy have been developed. In addition, some high dielectric constant liquid crystal materials have been developed, such as cyanogens crystal materials with two, three, or more fluoro-groups substituting for the cyanogen atoms. However, the impurity ion release rate of those fluoro liquid crystal materials with high dielectric anisotropy can be higher than the traditional liquid crystal materials with fluorine ion. Accordingly, it is desirable to provide some strategies for improving the reliability of the charge holding rate and for improving the mobility characteristic of the high-dielectric constant liquid crystal materials.
The first strategy involves improving the purity of the liquid crystal materials. However, the impurity of the liquid crystal materials can currently be rather high, and the possibility for further purifying the liquid crystal materials may be rather limited.
The second strategy is to eliminate the impurity released during the manufacturing processes and utilities. However, the manufacturing processes of the liquid crystal materials may be performed in a clean room that is cleaned regularly, so that the impurity released by the manufacturing environment is reduced.
The third strategy is to use liquid crystal materials with fewer ions. However, the choices of the liquid crystal materials are rather limited. In addition, the performance may be decreased. For example, the response time of the liquid crystal materials may be increased due to increasing the viscosity of liquid crystal materials, and/or, while the dielectric constant is decreased, increasing the drive voltage.
The forth strategy is to develop a sealing agent that allows the water and air flow through, instead of the impurity, such that the metal ion is prevented from passing through. Alternatively, a sealing agent may be developed that can release any impurity. However, for the moment, the development of the sealing agent has not been accomplished yet.
Japanese Patent Publication (Kokai) No. Heisei 10-81821 discloses a liquid crystal material, that can decrease the deviation of Pretilt Angle and the stretch of the alignment film due to the thermal treatment. Wherein the liquid crystal material is produced by mixing epoxy resin of the average molecular weight about 5,000 to 30,000 with the derivatives of polyimide and/or polyamic acid involved. The ratio of epoxy resin to polyimide and/or polyamic acid is ranged from 0.5:99.5 to 6:94. Although the Patent Publication (Kokai) No. Heisei 10-81821 can resolve the problems resulted from the thermo treatment, it does not resolve the degradation problems of the charge holding rate due to the release of ionic impurities.